Tumor Mutational Burden and response to immunotherapy

Tumor Mutational Burden as an Independent Predictor of Response to Immunotherapy in Diverse Cancers.
https://www.ncbi.nlm.nih.gov/pubmed/28835386


This study was a collaboration between UC San Diego, and Foundation Medicine.
Response rate, progression-free survival and overall survival following immunotherapies (in general, or specifically for immune checkpoint inhibitors such as anti-PD-1/PD-L1) was increased in patients with higher tumor mutational burden (as determined by genetic sequencing).


My commentary:
Newly diagnosed gliomas including GBM usually have low tumor mutational burden.  At highest risk for high tumor mutational burden (hypermutation) are likely those with MGMT-methylated tumors that recur following standard TMZ chemotherapy.

Pembrolizumab (Keytruda) is approved by the United States FDA, among other indications, "for the treatment of adult and pediatric patients with unresectable or metastatic, microsatellite instability-high (MSI-H) or mismatch repair deficient solid tumors that have progressed following prior treatment and who have no satisfactory alternative treatment options. "

The very high mutational burden sometimes seen in recurrent gliomas following temozolomide chemotherapy is often caused by mutations in one or more of the mismatch repair genes (MHS2, MSH6, MLH1, PMS2).  Having genetic sequencing performed (in North America by Foundation Medicine or Caris, or different companies in other countries) for recurrent post-temozolomide grade IV tumors (especially MGMT methylated tumors) could be a worthwhile way to detect hypermutation/mismatch repair deficiency/microsatellite instability and get access to pembrolizumab outside of a trial.  I can't comment on how easy getting coverage would be (both for the genetic testing as well as the pembrolizumab), as it would in part depend on the policies of the various insurance providers.